We have observed that the lumens of inverted follicles undergo periods of slow dilation followed by rapid shrinkage. The behavior is what would be expected from a closed follicle surrounded by cells that transport fluid into the lumen until a hole is produced through which the transported fluid leaks out rapidly. The hole seals, and the process is repeated. We have evidence that the microvilli-bearing surface of the inverted follicles has collagen receptors, although the surface never comes into contact with collagen normally. Separated thryoid follicles are unstable when embedded in a collagen gel. Single cells migrate away from many of these follicles. The migratory cells appear to be of epithelial origin from studies using labeled antibodies.